Research was performed at Crag Cave, Castleisland, southwest Ireland, and P8
Cave, Castleton, Derbyshire, in order to determine the main factors responsible for
modifying rainwater geochemistry during flow through soil and karstic aquifer
zones. Monitoring was performed on a daily basis in summer and winter at Crag
Cave, and on a monthly basis over one year at P8 Cave. At both sites, biannual
peaks in karst system Ca2+concentrations occurred due to: (i) promotion of
microbial C02 production by increased summer temperatures, and (ii) retardation
of gaseous exchange by ponding of elevated winter rainfall input leading to an
unseasonable build up in soil zone C02. Therefore, speleothems at both sites
may form biannual bands in hydrological years subject to elevated winter rainfall
input.
In addition to variations in carbonate weathering due to fluctuations in C02 levels,
cation yields in Crag Cave matrix soil water were controlled by dolomite dissolution
(Mg2+), plant uptake (K+), and evapotranspiration balanced by enhanced winter
marine aerosol input (Na+). Strontium isotope analysis indicates that S~+ was
derived from a 50:50 silicate/carbonate mixture, whilst the relatively light 313C
signal was related to direct evolution of C02 into the aqueous phase in waterlogged
pores.
Within the Crag Cave aquifer variations in karst water geochemistry were
controlled by dilution, flow switching, prior precipitation of calcite and dolomite
dissolution along the flow path. Strontium isotope analysis indicates that dissolution in the aquifer dominated, with S~+ being sourced from a 25:75
silicate/carbonate mixture. Light karst water 313C values were constrained by the
supply of light soil gas to the aquifer.
Elevation in the Mg/Ca and Sr/Ca ratios in the Crag Cave speleothem record
compared to present day analogues indicates that the former Holocene climate
was drier, whilst heavier 87Sr/86Srratios and 813C values suggest variation in soil
hydrology over time.